The present invention is directed generally to transfer cases for four-wheel drive automotive vehicles and more particularly to an apparatus and method for converting a full-time four-wheel drive transfer case for part-time four-wheel drive operation.
Prior to approximately 1974, most four-wheel drive vehicles were equipped the same. The front drive mechanism of the four-wheel drive vehicles consisted of a rigid front axle housing which was quite similar to the rear axle housing manufactured by American automakers. A U-joint or knuckle at both ends of the axle housing allowed the wheels to turn left and right. Attached to each knuckle was a hub to which the wheel was bolted and a wheel bearing protruded externally through the wheel. At the very tip of each end of the axle was a mechanism known as a "lock-out" hub. This lock-out could be positioned in either a locked or free position. In the free position, the wheel and hub rotated independently of the axle which is located inside the axle housing. In the locked position, the wheel and hub were secured to the axle which in turn was connected to the differential located in the center of the axle housing. The differential was connected to the front drive shaft which in turn was connected to the transfer case.
The transfer case was connected directly behind the transmission for selectively transferring power from the engine and transmission to the rear drive shaft and the front drive shaft. The transfer cases were known as part-time transfer cases. In the two-wheel mode, and with the lock-outs in the free position, the vehicle would function as a normal two-wheel drive vehicle with all power being transferred to the rear drive shaft which in turn drove the rear axle and rear wheels. The front drive shaft, differential and axle would not move and would be in a stationary position. With the transfer case in the four-wheel mode and the lock-outs in the locked position, the front drive shaft, differential and axle would also pull the vehicle.
That type of four-wheel drive vehicle, with lock-outs and part-time transfer case, provided two major benefits. First, it was economical in that power and gasoline were not wasted in turning the front drive shaft, differential and axle when the extra traction of the front wheels was not needed. Secondly, wear and tear on the front driving parts was also minimized. The result was that the driver could use four-wheel drive when it was needed and turn off the front drive system when the rear wheels provided sufficient traction.
Sometime in 1974, however, the major American automakers introduced an entirely new system which used a full-time transfer case. In most cases, this transfer case is built by New Process Gear Division, a division of Chrysler Corporation. The New Process Model 203 full-time transfer case, for example, is a very elaborate mechanism. In addition to the typical gear system which provides a high and low range, the transfer case consists of a middle section which houses a large chain for driving the front drive shaft and a rear section which contains a compensating differential.
The differential works well in situations where all four tires of the vehicle have exactly the same amount of traction. However, when one wheel has less traction, for instance when one wheel is on ice and the other three are on dirt, the differential directs all of the power of the vehicle to the wheel on ice. This, of course, results in a loss of traction and movement. For the same reason, a vehicle equipped with a full-time four-wheel drive transfer case will not function with lock-outs on the front axle.
In order to compensate for this situation, the full-time transfer case is equipped with a locking mechanism which will lock out the differential and force both the front and rear drive shafts to move at the same speed. This, in essence, will guarantee that at least one wheel on each axle is rotated. In any event, whether the locking mechanism is engaged or not, the vehicle is in full-time four-wheel drive which necessarily results in increased wear and tear and decreased gas mileage.
Accordingly, a primary object of the invention is to provide an apparatus and method for converting a full-time four-wheel drive transfer case for selective part-time wheel drive and four-wheel drive operation.
Another object is to provide such an apparatus and method which will conserve fuel by permitting the front drive shaft, differential and axle of a vehicle to remain idle when the extra traction of the front wheels is not needed.
Another object is to provide such an apparatus and method which will minimize wear and tear on the front drive system of a vehicle by permitting the same to be disengaged at times.
Another object of the invention is to provide such an apparatus and method which involve a simple elimination and substitution of a minimum number of original parts.
Another object is to provide such an apparatus and method which is effective to convert a full-time transfer case so that it will operate like a part-time transfer case described hereinabove.
Another object is to provide an apparatus for converting a full-time transfer case for part-time operation, which apparatus is simple and rugged in construction and efficient in operation.
Finally, another object is to provide a method for converting a full-time transfer case for part-time operation, which method may be simply, quickly and easily performed.